1) Field of the Invention
The invention relates to a catalyst system for the preparation of 1,2-dichloroethane by reacting ethylene and chlorine, if desired in the presence of an inhibitor for reducing the formation of by-products, and to a process for the preparation of 1,2-dichloroethane using this catalyst system.
2) Description of the Related Art
The preparation of 1,2-dichloroethane (EDC) by reacting ethylene with chlorine in 1,2-dichloroethane as the solvent and reaction medium is known. The catalysts used to accelerate the addition reaction of chlorine with the ethylene molecule are, besides the chlorides of elements of main groups 3 to 6 and subgroups 1,4 and 6 of the Periodic Table, in particular anhydrous iron (III) chloride, since the latter is readily accessible and inexpensive (CA-A 689991, DE-C 640827 and DE-B 1768367). The principal by-products produced in this reaction are ethyl chloride, from the competing hydrochlorination of ethylene, and, with evolution of hydrogen chloride, 1,1,2-trichloroethane as a consequence of ethylene-induced substitution of the EDC formed.
the chlorination of ethylene is frequently also carried out in the presence of oxygen as a substitution inhibitor (US-A 2601322 and DE-A 1568583).
The addition reaction of chlorine with ethylene is carried out in industry both at reaction temperatures around the atmospheric boiling point of EDC, the heat of reaction liberated being utilized to distill off and purify by rectification the reaction product and possibly also crude EDC from other origins, and at lower temperatures of from 30.degree. to 60.degree. C., as described in Ullmanns Encyclopadie der technischen Chemie [Ullmann's Encylclopaedia of Industrial Chemistry], Volume 9, page 427 (1975). In the latter case, however, the temperature level of the reaction is kept so low that the reaction enthalpy liberated cannot conveniently be utilized, but instead must be dissipated into the cooling water or into the air by circulating the reaction medium by pumping via a heat exchanger (DE-A 1905517).
The crude, catalyst-containing EDC prepared at lower temperatures is usually discharged from the reactor in liquid form and must be washed with acidulated water to remove the catalyst and subsequently with aqueous alkali metal hydroxide solutions in order to neutralize the crude product. The phases are separated by decanting and separating off the aqueous layer, and the water-saturated crude EDC which remains is subsequently worked up in a known manner by distillation; however, this is complex.
In DE-A 2427045, ethylene is chlorinated at temperatures of from 100.degree. to 130.degree. C. and appropriately high pressures in order to be able to carry out the reaction in the liquid phase, in the presence of iron(III) chloride as catalyst. The product is then fed, with the circulating reaction medium, to a zone under lower pressure, in which the crude EDC formed is evaporated by the heat of reaction liberated during the reaction of chlorine with ethylene and is rectified with recycling of high-boiling impurities into the reaction zone. The result is an accumulation of high-boiling compound sin the reaction circuit. Although this accumulation can be kept within the desired limits by batchwise removal of a liquid catalyst-containing circulation product from the bottom of the reactor, it is, however, necessary to occasionally add iron(III) chloride to the reaction medium in appropriate amounts in order to prevent catalyst depletion. In addition, disposal of the discharge from the reactor bottom presents difficulties. Moreover, the use of iron(III) chloride as catalyst at elevated reaction temperatures is also associated with certain disadvantages. Firstly, iron(III) chloride promotes, at increasing temperature, the decomposition of the formed EDC with deposition of tar-like, amorphous and carbon-rich deposits (J. Soc. Chem. Ind. 69 (1950) page 289), which causes increased formation of byproducts with increasing temperature and, in particular, a considerable decrease in the ethylene yield. Secondly, iron(III) chloride has a corrosive effect in the presence of traces of water on the materials usually used in reactor and apparatus construction, which very generally causes an overproportional increase in the rate of corrosive attack with increasing temperature.
The corrosive behavior of iron(III) chloride is further increased by hydrogen chloride, which is virtually always present due to undesired side reactions, since the highly corrosive hydrogen tetrachloroferrate complex, which very easily releases aggressive protons, is thereby formed. For this reason, attempts have been made to carry out the reaction between ethylene and chlorine completely and selectively in the absence of metal salt catalysts by adding organic catalysts, such as, for example, hydroxyl-containing aromatic compounds (DE-B 1902843). However, this only succeeds for a short time, since, in particular at elevated temperatures, chlorination of the aromatic ring occurs with time and, on the other hand, the catalystic effectiveness of organic catalysts of this type drops drastically with increasing degree of chlorination.
The process of EP-A 82342 (iron(III) chloride catalyst in the presence of nitrogen bases, such as ammonia, amines or salts of these bases) or of EP-A 111203 (alkali metal or alkaline earth metal tetrachloroferrate catalysts) can be used to considerably reduce, in particular in the medium temperature range form 90.degree. to 120.degree. C., the corrosion in reactors made of conventional metallic materials which is caused by iron(III) chloride as catalyst in the preparation of EDC, if the iron(III) chloride catalyst is charged with the additives mentioned therein. In addition, these additives also have an advantageous effect on the formation of byproducts, which are thereby reduced in amount. However, at elevated reaction temperatures, which are unavoidable for economical utilization of the reaction enthalpy liberated during the chlorination of ethylene, these corrosion-inhibiting and reaction selectivity-promoting effects are considerably diminished since the ammonium or alkali metal or alkaline earth metal tetrachloroferrates which form on addition of ammonium chloride or organic amine hydrochlorides or alkali metal or alkaline earth metal chlorides to iron(III) chloride, are thermally labile and decompose more and more with increasing temperature to form the starting components, from which in turn extremely corrosive hydrogen tetrachloroferrate is produced to an increasing extent in addition to the virtually inert ammonium alkali metal or alkaline earth metal chlorides, which are now present and isolated as separate species. In the case of ammonium or amine hydrotetrachloroferrate complexes, this is supplemented by the fact that the ammonium ions are Bronsted acids, which eliminate highly corrosive protons with increasing temperature with formation of amine bases.
EP-A 113287 discloses a process in which about 85% of the heat of reaction liberated during the chlorination of ethylene in the presence of iron(III) chloride at temperature of from 140.degree. to 180.degree. C. can be utilized to generate steam. In this process, the EDC leaving the reaction zone is cooled by heat exchange with water before further work-up or recycling into the reaction zone. Apart from the negative corrosion behavior of bare iron(III) chloride, many byproducts are also produced. However, it is certainly the more correct way industrially to utilize the reaction enthalpy liberated to generate steam or to heat a heat-transfer medium, since this is more flexible and optimum recovery of the reaction enthalpy from the chlorination of ethylene can be achieved, even in the case of "unbalanced" conditions which occur during daily events or in general.
In the process of EP-A 75742, the reaction mixture form the chlorination of ethylene is divided into two sub-streams, or which one is passed through a heat exchanger and then returns into the circuit, while the second sub-stream is depressurized and fed to a rectification column, which is heated via said heat exchanger. In this procedure, there is none of the flexibility just discussed, so that deviations from the "balanced process", due to the lack of sufficient heat of reaction to be liberated or due to production of EDC in excess of that from the "balanced process" mean that the high-boiling component column integrated into the reaction system must additionally be heated with steam or, in the reverse case, excess reaction enthalpy must be passed into the cooling water or into the air via a trim cooler. Deviations from the "balanced process" thus occur, for example, if, due to a temporary lack of chlorine, the chlorination of ethylene must be reduced or if, due to an oversupply of external hydrogen chloride, the oxychlorination of ethylene is enhanced or, in the normal direct chlorination procedure, the oxychlorination alternatively operates at a lower rate due to reduced production of vinyl chloride monomer.
The object was therefore to develop a catalyst system for the preparation of 1,2-dichloroethane which ensures the highest possible selectivity and yield in all industrially important temperature ranges, i.e. from 0.degree. C. to 300.degree. C., in the chlorination of ethylene, and a minimum of corrosion.
A further object was to convert this catalyst system to a simple and economical process for the preparation of EDC by chlorinating ethylene, which process makes it possible, depending on the local conditions, either to sue at least some of the reaction enthalpy liberated for isolating pure EDC of cracking quality or to utilize virtually all the reaction enthalpy liberated to generate steam at an industrially useful pressure level; the catalyst system should remain in the reaction medium, and the EDC formed need only be freed from small amounts of high-boiling impurities before being used in a pyrolysis furnace.